In this paper, we propose an energy and cost efficient WDM/OFDMA PON system, referred to as NEWOPS. Based on a new two-tiered semi-tree architecture, NEWOPS allows a large number of ONUs to be connected to OLT using only a handful of colorless transceivers. Due to the architecture, NEWOPS not only facilitates high spectral efficiency and flexible sub-channel bandwidth allocation, but also reduces the Rayleigh backscattering (RB) problem and achieves high energy and cost efficiency. Experimental results demonstrate that, taking only 5-GHz bandwidth for each wavelength, NEWOPS successfully achieves 20-Gb/s downstream and 10-Gb/s upstream OFDM-xQAM transmissions via the same fiber trunk. We depict that, to support 40 ONUs for example, NEWOPS achieves energy efficiency that is three to six times higher than that of a typical 10-Gb/s WDM/OFDM PON.